
	March 31, 2020
	Amortized Cost Basis		Gross Unrealized Holding G ains		Gross Unrealized Holding Losses			Estimated Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	10,039	$	74	$	—		$	10,113
Obligations of states and political subdivisions		1,731,392		78,996		(2,717	)		1,807,671
Residential mortgage-backed securities		1,617,341		62,659		(20	)		1,679,980
Commercial mortgage-backed securities		587,114		17,624		(184	)		604,554
Corporate bonds and other		4,624		133		(6	)		4,751

Total securities available-for-sale	$	3,950,510	$	159,486	$	(2,927	)	$	4,107,069


	December 31, 2020
	Amortized Cost Basis		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Estimated Fair Value
Securities available-for-sale:
Obligations of states and political subdivisions	$	2,283,616	$	143,339	$	(79	)	$	2,426,876
Residential mortgage-backed securities		1,421,922		50,473		(115	)		1,472,280
Commercial mortgage-backed securities		467,243		22,077		(4	)		489,316
Corporate bonds and other		4,398		159		—			4,557

Total securities available-for-sale	$	4,177,179	$	216,048	$	(198	)	$	4,393,029

The Company did not hold any securities classified as

held-to-maturity

at March 31, 2021, March 31, 2020, or December 31, 2020.

The Company invests in mortgage-backed securities that have expected maturities that differ from their contractual maturities. These differences arise because borrowers may have the right to call or prepay obligations with or without a prepayment penalty. These securities include collateralized mortgage obligations (CMOs) and other asset backed securities. The expected maturities of these securities at March 31, 2021 and 2020, and December 31, 2020, were computed by using scheduled amortization of balances and historical prepayment rates.

The amortized cost and estimated fair value of

available-for-sale

securities at March 31, 2021, by contractual and expected maturity, are shown below (in thousands):


	Amortized Cost Basis		Estimated Fair Value
Due within one year	$	290,536	$	295,145
Due after one year through five years		2,355,100		2,456,956
Due after five years through ten years		2,145,293		2,188,324
Due after ten years		170,509		169,206

Total	$	4,961,438	$	5,109,631

The following tables disclose the Company’s investment securities that have been in a continuous

unrealized-loss

position for less than 12 months and for 12 or more months (in thousands):


	Less than 12 Months				12 Months or Longer				Total
March 31, 2021	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Obligations of states and political subdivisions	$	533,213	$	8,198	$	—	$	—	$	533,213	$	8,198
Residential mortgage-backed securities		589,353		8,618		999		10		590,352		8,628
Commercial mortgage-backed securities		471		—		—		—		471		—
Corporate bonds and other		32,490		1,539		—		—		32,490		1,539

Total	$	1,155,527	$	18,355	$	999	$	10	$	1,156,526	$	18,365


	Less than 12 Months				12 Months or Longer				Total
March 31, 2020	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Obligations of states and political subdivisions	$	168,725	$	2,717	$	—	$	—	$	168,725	$	2,717
Residential mortgage-backed securities		1		—		8,164		21		8,165		21
Commercial mortgage-backed securities		44,724		173		9,630		10		54,354		183
Corporate bonds and other		220		6		—		—		220		6

Total	$	213,670	$	2,896	$	17,794	$	31	$	231,464	$	2,927

	Less than 12 Months				12 Months or Longer				Total
December 31, 2020	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Obligations of state and political subdivisions	$	25,214	$	79	$	—	$	—	$	25,214	$	79
Residential mortgage-backed securities		36,017		96		3,156		19		39,173		115
Commercial mortgage-backed securities		16,218		4		—		—		16,218		4

Total	$	77,449	$	179	$	3,156	$	19	$	80,605	$	198

The number of investments in an unrealized loss position totaled 149 at March 31, 2021. Any unrealized losses in the obligations of state and political subdivisions, residential and commercial mortgage-backed and asset-backed investment securities at March 31, 2021 and 2020, and December 31, 2020, are due to changes in interest rates and not credit-related events. As such, no allowance for credit losses is required at March 31, 2021 and 2020, and December 31, 2020. Unrealized losses on investment securities are expected to recover over time as these securities approach maturity. Our mortgage related securities are backed by GNMA, FNMA and FHLMC or are collateralized by securities backed by these agencies. At March 31, 2021, 77.27% of our

available-for-sale

securities that are obligations of states and political subdivisions were issued within the State of Texas, of which 52.70% are guaranteed by the Texas Permanent School Fund.

At March 31, 2021, $3,103,077,000 of the Company’s securities were pledged as collateral for public or trust fund deposits, repurchase agreements, a borrowing line with the Federal Reserve Bank of Dallas and for other purposes required or permitted by law.

During the three-months ended March 31, 2021 and 2020, sales of investment securities that were classified as

available-for-sale

totaled $10,631,000 and $95,437,000, respectively. Gross realized gains from security sales during the first quarter of 2021 and 2020 totaled $808,000 and $2,062,000, respectively. There were no gross realized losses from security sales during the three-month periods ended March 31, 2021 and 2020, respectively.

The specific identification method was used to determine cost in order to compute the realized gains and losses.